Study of heavy ion induced fission in a lighter actinide 227Pa using NAND facility

Abstract

Over the decades of intense studies in the fission of actinide nuclei, it has been established that the fragment mass distribution is asymmetric at lower excitation energies (E*). The macro- microscopic model describes it as a manifestation of fragment shell effects [1]. With an increase in nuclear excitation energy, the shape of the mass distribution changes from asymmetric to symmetric Gaussian, pointing to the attenuation of shell effects with the increase of excitation energy. Surprisingly, recent experimental results of mass distribution studies stand remarkably far from these expectations [2]. In a seminal work, Hirose et al. studied the fission of neutron-rich actinides formed through multinucleon transfer (MNT) [2]. They observed the presence of shell-mediated asymmetric fission even above E* ≈ 60 MeV in a series of neutron-rich actinide nuclei. Subsequent theoretical calculations incorporating multi-chance fission have yielded reasonably accurate mass spectra. The primary focus of the investigations presented in this thesis is to examine the influence of multi-chance fission on the decay of compound nuclei (CN) formed through the conventional full- momentum transfer (FMT) fusion process. To address the phenomena of multi-chance fission, we have investigated the mass distribution and neutron multiplicity in an actinide nucleus at various excitation energies. Detection of fast neutrons coinciding with fission events was accomplished using the National Array of Neutron Detectors (NAND) facility [3]. NAND is a multi-neutron detector facility comprising one hundred BC501A organic liquid scintillators mounted on a geodesic dome structure and installed at the Inter-University Accelerator Centre (IUAC) in New Delhi for the study of heavy ion-induced fission and associated phenomena. The thesis also provides a comprehensive overview of the facility and includes Monte Carlo calculations performed to assess its performance characteristics. Following optimal tuning and performance evaluation, the neutron array has been utilized for heavy ion-induced fission research. In the experimental approach, we conducted measurements of the mass distribution, average neutron multiplicity, and correlations between fragment mass and pre-scission neutron multiplicity in the 227Pa compound nucleus formed through the complete fusion reaction 19F+208Pb, in the excitation energy range of ≈ 30 to 60 MeV. To analyze the data, we employed well-known methods such as the velocity reconstruction method for deriving the mass distribution and the moving source fit method for determining neutron multiplicity. The results were analyzed theoretically using GEF model calculations [4], incorporating multi-chance fission. Analysis of the Mass−TKE correlation spectra at higher energies suggests that multi-chance fission does not significantly influence the fission modes. However, at energies ≈ 35 MeV, the measurements have shown evidence of asymmetric fission. Theoretical analysis at these two energies reveals a significant occurrence of multi-chance fission and resultant asymmetric fission driven by fragment shells. Thus, the observed trend in Mass−TKE distribution is attributed to the presence of shell-influenced asymmetric fission modes facilitated by higher chance fission. This conclusion is ascertained by Mass−νpre correlation measurements performed at these energies. In conclusion, our findings suggest that the correlation of higher pre-scission neutron multiplicity with asymmetric fission is a signature of shell effects reinstated by multi-chance fission.